Conventionally, adhesive strength is optimized when the surfaces of the underlying structure has been cleaned to remove environmental contaminates, such as dirt, oxidation, and other debris immediately prior to bonding. With many manufacturing processes, however, the cleaning and bonding operations are often staged so that they are separated significantly in time, during which time the surface may become re-contaminated, thus lessening the adhesive strength of the bond to be formed.
To alleviate this concern, the cleaned surface may be primed with a primer. In the past, primers have often been prepared from one or two component thermosetting resins, typically diluted with organic solvents to allow for spray application. Liquid epoxy esters were proposed for use in coatings over forty years ago, in compositions containing either water or the epoxy ester itself as a carrier vehicle, together with polyvinylacetate, polyacrylic, or poly(butadiene/styrene).
One such example of these organic solvent based primers includes U.S. Pat. No. 4,352,899 (Tada), in which is described a coating composition for metal substrates of an epoxy resin, an organic phosphorus compound, zinc powder, and a magnesium compound dispersed in a liquid medium. A portion of the zinc powder may be replaced by a solid electroconductive material. These compositions are applied in organic solvents, and the compositions may contain various pigments including chromate pigments.
In an effort to address environmental and regulatory concerns over the use of organic solvents, two component primer systems have been designed and developed in which one component is an epoxy resin dispersion in water, either with a reduced volume of solvent or not at all, together with various corrosion inhibitors, mainly chromate based ones, and the second component is a reducible amine catalyst in water.
For instance, U.S. Pat. Nos. 5,461,090, 5,576,061, and 5,641,818, each disclose an aqueous, essentially VOC-free adhesive epoxy primer for promoting the bonding of a metal adherend to another adherend without reported loss of physical properties, in a storage stable environment, and exhibiting solvent resistance. The primer disclosed by these patents is prepared by dispersing one or more solid epoxy resins and a solid curing agent into water such that 100% of the solid particles of epoxy resin and curing agent have a size of less than 30 .mu.m.
Further environmental and regulatory issues have developed recently over the use of chromates, and in particular the use of chromates as corrosion inhibitors. In the past, various types of liquid coating compositions have been applied to metallic substrates and baked thereon in order to protect the substrates against corrosion. Certain of such coatings are applied in conventional metal coil coating processes, and they must be sufficiently adherent and flexible to resist cracking, chipping and peeling. One process which has been utilized for improving the corrosion-resistance of various metal substrates generally involves the application of two coatings. The first coating is comprised of a material, such as xanthan gum, as a carrier for the other ingredients of the coating which include a chromium compound such as chromium trioxide and zinc dust. On baking, the xanthan gum contained in the coating becomes water-insoluble. Over this baked coating is applied a second coating which comprises a zinc rich resin. U.S. Pat. No. 4,026,710 (Kennedy) describes such two-step procedures for improving the corrosion-resistance of metal. In this case a special tenside is used in order to get the resins dispersed in water.
U.S. Pat. No. 3,713,904 (Bernath) describes compositions for producing corrosion-resistant and protective coatings on aluminum and aluminum alloys, which are based on an organic resin, an inorganic hexavalent chromium compound, an oxidizable component, phosphoric acid and strontium chromate. On mixing, the strontium chromate and oxidizable component react to reduce the hexavalent chromium to trivalent chromium. The mixture is applied to the substrate, which is then heated to oxidize a portion of trivalent chromium to hexavalent chromium resulting in a strongly adherent organic resin coating.
And U.S. Pat. No. 5,859,095 (Moyle) claims an aqueous composition of a water-dispersible or emulsifiable epoxy resin together chromium trioxide, water, phosphoric acid and a polytetrafluoroethylene lubricant.
One approach to the elimination of chromate in corrosion inhibitor packages involved the use of zinc. Zinc-rich coatings useful for improving corrosion-resistance are described in U.S. Pat. No. 4,476,260 (Salensky). These coatings include generally a mixture of zinc pigment, thermoplastic epoxy resins, an organosilane and optionally aluminum trihydrate and one or more dispersing agents. A washcoat composition suitable for application to tin-plated mild steel is described in U.S. Pat. No. 4,544,686 (Bromley), which includes an aqueous carrier medium and a binder. The washcoat composition includes a thermosetting acrylic polymer, an epoxy resin and an acid catalyst.
In addition, U.S. Pat. No. 6,139,610 (Sinko) claims a corrosion-inhibiting composition for application to a metal substrate. The composition so claimed is a film-forming organic polymer which has dispersed therein a corrosion inhibiting pigment phase, which itself is a composite organic/inorganic hybrid microparticle formed of an inorganic corrosion inhibiting compound which is interfaced on a microscopic level with micro-particles formed of an organic corrosion-inhibiting compound. These inorganic and organic compounds are said to be distinctly identifiable by spectrum analysis, but physically combined into inseparable component phases having different chemical compositions.
U.S. Pat. No. 5,866,652 (Hager) relates to a coating on a metallic skin of an aircraft. The coating includes a continuous phase of an organic polymer or sol-gel, and a mixture of chromate-free salts dispersed throughout the continuous phase. The mixture of chromate-free salts includes carboxylate salts of rare earth metals and vanadate salts of alkali or alkali earth metals; and a borate salt of an alkali earth metal. The coating is reported to protect the metallic skin from significant pitting corrosion after 3,000 hours of exposure to a spray of 5 wt % sodium chloride solution. The coating is reported to be resistant to degradation by aircraft hydraulic fluid. In addition, the coating is reported to be able to withstand a 50-inch-pound forward impact delivered by a Gardner 160 pound capacity test machine.
Aqueous primer compositions containing substantially no volatile organic solvents are described in U.S. Pat. No. 5,461,090 (Sweet). The composition may also contain non-chromate corrosion inhibitors, preferably such as zinc phosphate and zinc molybdate.
In addition U.S. Pat. No. 6,537,678 (Putnam) reports of non-carcinogenic corrosion inhibiting additives. These additives include an anodic corrosion inhibitor and cathodic corrosion inhibitor, where the anodic corrosion inhibitor is selected from compounds of vanadium, molybdenum, tungsten and the cathodic corrosion inhibitor is selected from compounds of cerium, neodymium, and praseodymium.
Self-dispersing curable epoxy resins for coatings are disclosed in U.S. Pat. No. 6,506,821 and international Patent Publication No. WO 96/20971. In U.S. Pat. No. 6,506,821 (Huver), self dispersible curable epoxy resin composition are obtainable by the reaction of 1.0 equivalent of epoxy resin, 0.01 to 1.0 equivalent of a polyhydric phenol and 0.005 to 0.5 equivalent of an amine-epoxy adduct, the amine-epoxy adduct being a reaction product of an aromatic polyepoxide with a polyalkyleneamine. The aromatic polyepoxide and the polyoxyalkyleneamine are used in an equivalent ratio of 1:0.1 to 1:0.28.
In U.S. Pat. No. 5,648,409 (Katar), self-dispersing curable epoxy resin compositions are prepared by reacting 1.0 equivalent of epoxy resin, 0.01 to 1.0 equivalent of a polyhydric phenol and 0.005 to 0.5 equivalents of an amine-epoxy adduct, the amine-epoxy adduct being a reaction product of 1.0 equivalent of an aromatic polyepoxide and 0.3 to 0.9 equivalent of a polyalkyleneamine.
The international Patent Publication No. WO 07/008199 discloses a VOC free aqueous-based primer compositions comprising a thermosetting resin composition, a corrosion inhibitor, water and a curative for adhesive bonding systems and coatings, especially to inhibit corrosion. The inventive primer compositions are substantially free of chromate but always comprise corrosion-inhibitors, like organic salts, pigments or anodic and cathodic inhibitors. Preferably these inhibitors have a particle size which is less than the primer layer thickness of a coating to provide less corroding surface. The production of particles of this size is expensive and requires special apparatus and trained personnel.
Notwithstanding the state of the technology, it would be desirable to create alternative substantially VOC free primer for producing corrosion-resistant and protective coatings and adhesive bonding systems that are prepared with substantially no chromate component; are substantially free of additional corrosion inhibitors or only comprise a reduced amount of said corrosion inhibitors and still inhibit corrosion; can easily be adapted to different application profiles and when cured, exhibits resistance to organic solvents and corrosion, so that the end user has a variety of commercial sources of advanced primers from which to choose.
Further, it would be desirable to provide a coating, a primer and sealant composition for application to metal surfaces for protecting exposed surfaces against corrosion.